Conventionally, a carbon tool steel strip suitable for use in springs, valves, or the like is rolled to a predetermined thickness, then quenched and tempered to adjust target properties, and then processed into a target shape for use by e.g. blanking.
For carbon tool steel used after quenching and tempering in this manner, JPA-2006-63384 (Patent Literature 1), for example, proposes to improve impact properties by controlling an amount of residual carbides in a metallographic structure of the carbon steel. The proposed high carbon steel member has the improved impact properties by controlling a volume % (Vf) of unsolved carbides in a matrix within a range of 8.5<15.3×C %−Vf<10.0.